Frequency stabilized oscillator



p 1947. R. HARVEY 2,427,498

FREQUENCY STABILI ZED OSCILLATOR Filed Sept. 27, 1945 '2 Sheets-Sheet 1FREQUENCY DEV/4770A! IN CYCLES FROM If MEGAC'YCLFS 20 40 60 80 I I I I IANODE VOLTAGE INVENTOR ROBERT L. HARVEY ATTORNEY Sepf. 16, I947. R.HARVEY FREQUENCY STABILIZED OSCILLATOR Filed Sept. 27, 1943 2Sheets-Sheet 2 INVENTOR ROBERT L. H RVEY BY W ATTORNEY Patented Sept.16, 1947 FREQUENCY STABILIZED OSCILLATOR Robert L. Harvey, Princeton, N.J., assignor to Radio Corporation of America, a corporation of DelawareApplication September 27, 1943, Serial No. 503,962

4 Claims. 1

This invention relates to oscillation enerators and has for itsprincipal object to provide an electron oscillator which is tunable overa wide rang of frequencies and which is adapted for operation at astabilized frequency.

My invention is the outgrowth of certain research work which wasundertaken with the object of providing a generator the frequencystability of which would be maintained within .02% under practicaloperations conditions, the use of quartz crystals being purposelyavoided in order to cover the required tuning range.

It is another object of my invention to provide means and a method ofstabilizing the frequency of an oscillator against supply voltagevariations and against changes in the circuit parameters.

due to aging and other factors.

As is well known, there is a considerable frequency drift in oscillatorswhich is most pronounced during the warm-up period. Frequency drift isalso attributable to mechanical expansion, distortion, or buckling ofthe tube elements and secondary emission from the bulb and the micaparts Within the tube. While some of these factors are not easilycompensated, I have found that there is a great advantage to be derivedfrom the use of cores in the inductive elements of the tuned circuit.Preferably, I employ two such cores, one being made of copper, and theother of a material generally known as magnetite. Powdered iron ormagnetite cores are well known in the art, but I have discovered whatare believed to be novel methods of using them to compensate forfrequency drift caused by changes in the circuit parameters, due, amongother factors, to variations in the supply voltage.

My invention will now be described in more detail, reference being madeto the accompanying drawings, in which:

Fig. 1 shows diagrammatically a conventional Colpitts oscillator circuitarrangement employing adjustable cores in association with the tunedinductance of the circuit;

Fig. 2 shows a circuit diagram similar to that of Fig. 1 but followingthe teachings of Hartley;

Fig. 3 shows graphically the correlation between anode voltagevariations and frequency derivations, the several curves being plottedunder different conditions of adjustment of a magnetic core within thetuning coil;

Fig. 4 shows a cross-sectional view through the tuned circuit andmounting therefor in an oscillator, certain features of my inventionbeing therein detailed; and

Fig. 5 shows an outline of the tuning condenser plates.

Referring first to Fig. 1, I show a conventional Colpitts oscillatorcircuit arrangement which is one of two alternative circuits preferablyemployed in carrying out my invention. This circuit includes an electrondischarge tube I having a cathode 2, a grid 3, and an anode 4. A sourceof direct current operating potential indicated by -B and +13 terminalsis connected between the cathode 2 and the anode 4. A tuned circuitcomprising an inductance 5 and adjustable capacitor 6 is directlyconnected at one terminal to the anode 4. The other terminal is coupledto the grid 3 by way of capacitor 1. Capacitor 8 intercouples grid 3 andcathode 2. A grid leak resistor 9 is in shunt with capacitor 8. Thepositive terminal of the direct current source is connected to the anode4 through a choke l0,

Fig. 1, as a circuit arrangement, is not claimed to be novel, except forthe fact that I have provided a novel frequency stabilizing meanscomprising adjustable copper and magnetite cores and a combination ofthe same. One of these cores II is preferably made of copper while theother core I2 is preferably made of comminuted iron, or particles ofmagnetic material known in the art as magnetite and held together bymeans of a binder. The method of adjusting these cores II and [2 for thepurpose of gaining frequency stability within a given range of anodevoltages while tuning the tank circuit to a desired frequency will behereinafter set forth in more detail.

The circuit arrangement of Fig. 2 is substantially a Hartley oscillator.It may employ the same tube l as shown in Fig. 1, this tube having thesame electrodes 2, 3, and 4 .as shown in Fig. 1. The principaldifierence between the Hartley oscillator and the Colpitts oscillator,as is well known, resides in the use by the Hartley oscillator of atapped inductance in place of series capacitance for deriving a suitablyphased feedback potential. Somewhere near the mid-point of theinductance 5a a tap I3 is provided for coupling across capacitor M tothe cathode 2. Another tap 15 on the inductance 5a is used for couplingthe inductance across capacitor I6 to the grid 3. In the particularembodiment of the circuit which I have used for working in a band offrequencies adjacent to II megacycles, the inductance 5a consisted offour turns from the anode end 'down to tap [3, two turns between taps l3and I5 and two more turns from tap [5 to the lower end of the coil.

Fig. 4 shows mechanical details of construction of the oscillator whichI preferably use. In order to avoid frequency drift and heating of thecomponent circuit parts, the tube I is preferably mounted at the end ofwire rods l1, about 3" long and having pins (not shown) adapted forinsertion in a conventional tube socket l8. Socket I8 is supported by abracket l9 within a cup-shaped housing 20, the latter being of metal inorder to provide suitable shielding.

A head 2| is provided for the cup-shaped shield member 20 and may besecured thereto in any. convenient manner, such as by means of screws30. This head supports the bracket l9 and also a tubular member 21 madeof insulating material on which the condenser plates 6 and theinductance coil 5 are mounted. The lower end of the insulating tube 2'!is slipped into a hole in the bottom of the cup 21) when assembling. Forconvenience in assembling, the insulatingtube 2! is seated in the head2| before insertion into the cup-shield and is secured by means ofwashers 22 and 23 which are clamped together by means of screws 3| andnuts 32 on the two sides respectively of the head member 2|. The members23 are half-washers each, since their inner circumference is too smallto go over a shoulder portion 21a on the insulating tube 21. Thisshoulder is provided for holding the condenser plates 6 in place. Aspacing washer 28 of insulating material holds the two condenser platessuitably spaced apart. A retaining ring 29 of resilient material fitsinto a groove in the insulating tube 2'1. This construction facilitatesthe necessary tuning adjustment of the condenser plates 6 since eitherof these plates may be rotated with respect to the other. Theirsemicircular outline is shown in Fig. 5 and provides for varying thecondenser plate areas which may be mutually opposed.

The tuning inductance 5 is wound on the lower portion of the insulatingtube 21. The connections between the terminals of the inductance coiland the respective plates may be made in any convenient manner. Theseconnections are not, therefore, shown in Fig. 4 nor are the connectionsindicated to the terminals of the socket it, since they would undulycomplicate the draw ing. Reference is made to Figs. 1 and 2, however,for two alternative circuit arrangements.

Secured to the washer plates 22 at thetwo ends of the assembly arestationary micrometer sleeves 26. These sleeves fit into micrometerheads 24 and the latter are provided with rotatable shafts 25 and 25a.The sleeves ZB may be threaded either externally or internally. Matingthreads are, therefore, provided either internally of the micrometerheads 24 or' externally of the shafts 25, 25a. The screw threads are notshown because concealed within the heads 24;

On the end of shaft 25 is mounted a copper core II, while on the end ofshaft 25a is m'ounteda magnetite core 12.

The stationary sleeves 26 are axially graduated while the knobs 24 aregraduated around the circumferences of their bevelled ends. Each of theknobs may be turned to any desired point for raising or lowering thecore members II and I2 respectively. The graduations of the elements 24and 25 enable the adjustment of these core members to be made within anaccuracy of .001.

Referring now to Fig. 3, there is therein shown a family of curvesrepresenting frequency deviations from a normal tuning of an oscillatorat 11 megacycles when the anode voltage is varied within certain limits.Different curves A to E inclusive are plotted to show the effect ofinserting the magnetite core by successive steps into the magnetic fieldof the tuning inductance 5. Such steps are arbitrarily chosen for thesake of illustration. The curve A represents a minimum practical limitof insertion of the mag netite core within the inductance, and curve Erepresents an approach to a maximum practical limit of insertion of thecore. The adjustment of the copper core II is used primarily for tuningpurposes.

The frequency deviations from 11 megacycles (an operating frequencychosen merely for purposes of illustration) are shown by the verticalscale at the left of the chart. The horizontal scale representsvariations in anode voltage. It should be noted that when the magnetitecore adjustment is made according to curve A, there is very littlefrequency variation within a range of 160 to 180 volts. A frequencyvariation when the core is adjusted as in accordance with curve B isvery slight within the range of to 160 volts. Curve C shows a minimumfrequency variation within the voltage range of 80 to volts. If it isdesired to operate within an anode voltage between 60 to 80, then thecore adjustment is preferably made according to curve D, and an evenlower voltage without serious frequency variation is practical if thecore adjustment is in accordance with curve E.

From the foregoing description of the chart of Fig. 3, it will be clearthat considerable frequency stability and freedom from frequency driftis obtainable by means of the magnetite core adjustment for working indifferent anode voltage ranges. Furthermore, to compensate for thechange in frequency attributable to the adjustment of the magnetite corel2, the copper bore ll may be adjusted to restore the tank circuit tothe required normal frequency.

It has been found in practice that frequency deviation due to changes inthe filament voltage are very slight in comparison with frequencydeviation attributable to anode voltage variations. Accordingly, I havenot herein shown the effects of filament voltage variations. It is well,however, to recognize that such effects exist in a minor degree but theyare not so serious but that they may be neglected in viewof thecompensation provided by the core members II and I2 and the adjustingmeans therefor.

It will be appreciated by those skilled in the art that in an oscillatorthe tube characteristics are mainly responsible for instability of thefrequency. The changes in the input and output resistances, however,increase the difiiculty of maintaining stability. A change in theseresistances caused by variations in plate voltage or cathode emissionwill shift the phase of the regenerated oscillation. This calls forcompensating phase shift in the resonant circuit. The use of-copper andmagnetite cores in association with the inductance of the resonantcircuit, I have found to be very advantageous in that the compensationso provided is such as to shift the frequency of the resonant circuitoppositely to the frequency drift produced by the input and outputcircuit resistances and by other tube variables, especiallyduringvoltage changes of the supply source.

Another factor entering into the frequency stability problem is thechange of capacity between the tube elements with temperature changesand electron flow in the tube. These changes directly affect thefrequency to which the resonant circuit is tuned.

For best results in carrying out my invention, I have found that the Qof the resonant circuit should be made as high as practical. If aHartley oscillator is used, the taps l3 and i5 should be adjusted alongthe inductance Ed at points suitable for maintaining the feedback assmall as is consistent with dependable operation. I have found that agrid circuit load of 500 ohms and a plate circuit load of 2,000 ohms issatisfactory. I have also found that loose coupling between the tube andthe resonant circuit reduces frequency drift during the tube warm-upperiod. Low anode voltage also reduces the fre=- quency drift duringtube warm-up.

The value of the grid leak resistance 9 is not critical. A value of12,000 ohms has been found satisfactory when using tubes of types knownas RCA 955 and RCA 604.

A utilization circuit may be coupled to the oscillator eitherinductively or capacitively. Preferably, a buffer tube is used in orderthat the load may have a minimum effect on the oscillator frequency. Themaximum allowable voltage delivered to the buffer grid circuit dependson the anode voltage in the oscillator. With a hi h anode voltage, theoutput delivered to the grid of a buffer stage may be of the order of 1volt.

Numerous modifications of my invention will suggest themselves to thoseskilled in the art, but they should be understood to be comprehendedwithin the scope of the invention.

I claim:

1. An oscillation generator having a metallic housing, a tank circuitmounted within the housing, said tank circuit comprising parallel-tunedinductive and capacitive elements, a tubular support of insulatingmaterial for both elements of said tank circuit, a tube socket mountedwithin said housing, a discharge tube positioned externally of saidhousing for producing oscillations, the prongs of said tube havingindividual connections extending to the terminals of said socket,thereby to minimize the heating effects of said tube upon the contentsof said housing, a copper cylinder having a tuning function supplementalto that of the capacitive elements in said tank circuit, frequencystabilizing means including a core member containing magnetic particles,both said copper cylinder and said core member being adjustably disposedwithin said tubular support, and means external to said housing forindependently varying the positions of said cylinder and core memberalong the axis of said inductive element.

2. A generator according to claim 1 wherein the last said means includesmicrometer heads axially disposed at the two ends of said tubularsupport.

3. An oscillator comprising an electron discharge tube having at least acathode, an anode and a control grid, means including a tank circuitcoupled to the tube electrodes for determining the frequency of theoscillations generated, said tank circuit having a capacitor in parallelwith an inductive helix, a copper core member and a magnetic core memberadjustably posi tioned along the axis of said helix, a resistorconnected between the control grid and the cathode, a capacitor couplingsaid control grid to a suitable point on the helix of said tank circuit,a source of operating potential applied between the cathode and anode,an inductive impedance connected between the positive terminal of saidsource and the anode, the ohmic value of said impedance and the terminalvoltage of said source being suitably chosen to obtain a normal anodevoltage, compensating means operable by suitably positioning saidmagnetic core within the field of said helix so that the curve offrequency deviation with respect to anode voltage variation issubstantially flat at the normal anode voltage, and further compensatingmeans operable by suitably positioning the copper core member so as toobtain the desired tuning of the oscillator while operating at normalanode voltage.

4. An oscillator for generating oscillations of substantially constantfrequency despite changes over a limited range in the applied anodevoltage, comprising an electron discharge device having an anode, acathode and a control grid, a tank circuit having inductance andcapacity connected to said anode, cathode and control grid, said tankcircuit serving to determine the frequency of oscillations generated,the inductance of said tank circuit being established by a coil havingindependently adjustable copper and magnetic core members positionedtherein and movable along the longitudinal axis of said coil, a circuitfor subjecting said control grid to a suitable operating potential withrespect to said cathode, interconnections between said anode, cathodeand control grid whereby oscillations are regeneratively set-up at afrequency determined by the resonant characteristic of said tankcircuit, and a circuit for subjecting said anode to an operating voltageof suitably positive value with respect "to said cathode, the positionof said magnetic core within said coil being so chosen as to minimizefrequency variations due to changes in the anode voltage, and saidcopper core being so positioned as to effectively tune said system atthe desired operating frequency.

ROBERT L. HARVEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,690,228 l-Ieising Nov. 6, 19282,210,303 Polydoroff Aug. 6, 1940 FOREIGN PATENTS Number Country Date560,396 France July 9, 1923

